Recently, a circuit pattern width required for a semiconductor device becomes narrower with the progress of integration and capacity of a large scale integration (LSI). Using an original pattern (means a mask or a reticle, hereinafter collectively referred to as a mask) in which a circuit pattern is formed, the circuit is formed by exposing and transferring the pattern onto a wafer with a reduction projection aligner called a stepper, thereby producing the semiconductor device. An electron beam writing apparatus that can perform writing of the fine pattern is used in producing the mask used to transfer the fine circuit pattern to the wafer. The electron beam writing apparatus is also used in the case that the circuit pattern is directly drawn in the wafer.
The electron beam writing apparatus inherently provides a superior resolution, since an electron beam used for the electron beam writing apparatus is a charged particle beam. This apparatus is also advantageous in that great depth of focus can be obtained, which enables dimensional variations to be reduced even when a large step feature is encountered. A variably-shaped electron beam writing apparatus that is an example of the electron beam writing apparatus includes an electron gun that emits the electron beam, a first shaping aperture, a second shaping aperture, a shaping deflector, and some electron lenses that causes the electron beam to converge. The electron beam emitted from the electron gun is imaged on the first shaping aperture, and then imaged on the second shaping aperture. The electron beam is deflected with the shaping deflector, and a size and a shape of the electron beam are variably formed by optically superimposing a first shaping aperture image and a second shaping aperture image on each other. The shaped electron beam is shot on the mask that is the writing target, and shot graphics are accurately connected to each other to perform the writing of the pattern.
A thermoelectron emission type electron gun in which a cathode filament is used as a heater can be used as the electron gun of the electron beam writing apparatus. In the electron gun, electrons are emitted by heating a cathode by a filament power. The emitted electrons are accelerated by an acceleration voltage, and controlled by a bias voltage, and the mask is irradiated with the electrons with a predetermined emission current (See Japanese Patent Publication Hei 05-166481).
During writing operation, an area surrounding the electron gun becomes a high vacuum, a high voltage (acceleration voltage) is applied between the cathode and the anode, and the thermoelectron emission source included in the cathode is heated, thereby thermoelectrons are emitted from the thermoelectron emission source. The thermoelectrons are accelerated by the acceleration voltage and emitted as the electron beam.
Lanthanum hexaboride (LaB6) is well known as a material for the thermoelectron emission source. The lanthanum hexaboride (LaB6) has a high melting point and a low work function. The lanthanum hexaboride (LaB6) is relatively stable against residual gas, and has a longer life compared with the case that another material is used. Additionally, because the lanthanum hexaboride (LaB6) has an excellent ion impact resistance, the lanthanum hexaboride (LaB6) is used in not only the electron beam writing apparatus but also a thermoelectron emission emitter such as an electron microscope.
In the electron gun, there is a well-known technology for coating a thermoelectron emission source constituting material with a material having the work function larger than that of the thermoelectron emission source constituting material to restrict an emission area of the electron from the thermoelectron emission source in order to improve luminance. For example, P. B. Sewell et al., “Study on thermal emission in lanthanum hexaboride single crystal having fine plane”, Electron Optical Systems, pp. 163-170, SEM Inc., AMF O'Hare (Chicago), IL60666-0507, U.S.A. describes that lanthanum hexaboride (LaB6) is coated with carbon (C). As to the specific coating method, the carbon (C) is deposited on a surface of lanthanum hexaboride (LaB6) by a CVD (Chemical Vapor Deposition) method, a solution containing the carbon (C) is applied onto the surface of lanthanum hexaboride (LaB6), or the lanthanum hexaboride (LaB6) is dipped in the solution. After the coating, the lanthanum hexaboride (LaB6) is exposed from part of the carbon (C) by machine processing, and the electron is emitted through the exposed part.
Accordingly, preferably the conventional thermoelectron emission source constituting the cathode of the electron gun has a structure in which the constituting material such as the lanthanum hexaboride (LaB6) is coated with the carbon (C) layer.
FIG. 18 is a schematic sectional view of the conventional thermoelectron emission source.
As illustrated in FIG. 18, in a conventional thermoelectron emission source 100, surfaces of a cylindrical main body 101 and a conical tip 102 having a flat leading end are coated with a carbon (C) layer 103. For example, the main body 101 and tip 102 of the thermoelectron emission source 100 are integrally formed using the lanthanum hexaboride (LaB6).
The flat leading end of the tip 102 made of the lanthanum hexaboride (LaB6) is exposed from a tip portion of the thermoelectron emission source 100. The structure of the thermoelectron emission source 100 is formed by machine processing such as polishing as mentioned below.
FIG. 19 is a schematic sectional view illustrating a pre-machine processing state of the conventional thermoelectron emission source.
As illustrated in FIG. 19, a thermoelectron emission source constituting material 200 coated with a carbon (C) layer 203 corresponds to the thermoelectron emission source 100 in the pre-machine processing state, that is, before the machine processing such as polishing. For example, the thermoelectron emission source constituting material 200 is subjected to the polishing to become the thermoelectron emission source 100 in FIG. 1.
The thermoelectron emission source constituting material 200 includes a cylindrical main body 201 similar to the main body 101 in FIG. 18 and a conical tip 202 having a sharply pointed shape. The surface of the thermoelectron emission source constituting material 200 is coated with a carbon (C) layer 203. The main body 201 and tip 202 of the thermoelectron emission source constituting material 200 are similar to the main body 101 and tip 102 of the thermoelectron emission source 100 in FIG. 18. For example, the main body 201 and tip 202 are integrally formed using the lanthanum hexaboride (LaB6).
That is, the conical tip 202 of the thermoelectron emission source constituting material 200 has the sharply pointed shape, and the carbon (C) layer 203 coats the conical surface of the tip 202 and the side surface of the main body 201.
Using the thermoelectron emission source constituting material 200 coated with the carbon (C) layer 203, the conventional thermoelectron emission source 100 in FIG. 18 can be produced by the machine processing in which the tip portion of the thermoelectron emission source is polished with a polishing article. In this case, the post-machine processing coating layer 203 becomes the coating layer 103 of the thermoelectron emission source 100 in FIG. 18, and the post-machine processing tip 202 of the thermoelectron emission source constituting material 200 becomes the tip 102 having the flat leading end in FIG. 18. The main body 201 of the thermoelectron emission source constituting material 200 becomes the main body 101 in FIG. 18.
In a method for producing the thermoelectron emission source 100, during the polishing of the thermoelectron emission source constituting material 200 coated with the carbon (C) layer 203, a diameter of the portion exposed from the carbon (C) layer at the leading end of the tip 202 is checked with an optical microscope, and the polishing is repeated when the diameter of the portion exposed is smaller than a predetermined diameter value that is a target value. The conical tip 202 of the thermoelectron emission source constituting material 200 has a cone angle, and the diameter of the portion exposed from the carbon (C) layer, namely, the diameter of the lanthanum hexaboride (LaB6) increases gradually with the progress of the polishing.
In the thermoelectron emission source constituting material 200, the carbon (C) layer 203 is hard to form on the tip 202 made of the lanthanum hexaboride (LaB6) such that a layer thickness becomes uniform. As a result, it is difficult to predict the diameter of the portion exposed from the carbon (C) layer at the leading end of the tip 202 while the tip portion of the thermoelectron emission source constituting material 200 is sharpened by the polishing.
Therefore, as a result of the polishing, sometimes the diameter of the exposed leading end of the tip 102 of the obtained thermoelectron emission source 100, namely, the diameter of the lanthanum hexaboride (LaB6) is larger than the predetermined diameter value. In such cases, the diameter of the exposed leading end cannot be decreased by the similar machine processing such as the polishing. That is, in the method for producing the thermoelectron emission source 100, in the case that the leading end of the tip 102 is excessively polished by the machine processing in order to expose the leading end of the tip 102 from the carbon (C) layer 103, the leading end of the tip 102 is hardly repaired.
Accordingly, in the case that the tip is excessively polished in producing the thermoelectron emission source 100, it is necessary to discard the thermoelectron emission source 100 as a product that does not satisfy a specification. During the production of the thermoelectron emission source 100, in the case that the leading end of the tip 102 that is exposed from the carbon (C) layer and made of the lanthanum hexaboride (LaB6) is larger than the predetermined diameter value, it is necessary to discard the thermoelectron emission source 100. As a result, a yield is degraded in the production of the thermoelectron emission source 100.
In the case that the cathode of the electron gun is produced by incorporating the thermoelectron emission source 100 in the cathode, productivity of the cathode is degraded when the production yield of the thermoelectron emission source 100 is degraded.
Therefore, there is a demand for a yield improving technology in the method for producing the thermoelectron emission source constituting the cathode of the electron gun. Additionally, there is also a demand for a production efficiency improving technology in the method for producing the cathode used in the electron gun.
The present invention has been made in view of the above problems. Namely, an object of the invention is to provide a thermoelectron emission source producing method that improves the production yield.
Further, an object of the invention is to provide a cathode producing method that improves the production efficiency.
Other challenges and advantages of the present invention are apparent from the following description.